Photoluminescence Characterizes AlN

Researchers at Kansas State University in Manhattan have developed a means of characterizing the optical quality of AlN. The technique enables them to make semiconductor-quality AlN with the potential for fabricating deep-ultraviolet laser diodes.

A deep-UV laser spectroscopy system offers picosecond time-resolved photoluminescence measurements of AlN, enabling researchers to characterize and thereby improve the quality of the material. Atomic force microscopy images of this AlN crystal reveal a smooth surface morphology. The rms surface roughness is 0.8 nm for a 10 x 10-µm scan area (left) and 0.7 nm for a 2 x 2-µm area (right), comparable to high-quality GaN. Courtesy of Hongxing Jiang and Jingyu Lin.AlN has a number of desirable characteristics for photonic applications, including a high direct bandgap of approximately 6.1 eV at room temperature, high thermal conductivity, hardness and chemical resistance. Unlike many other III-nitride compounds, however, measuring the optical and electrical properties of AlN has been a challenge because the material is a good insulator, which renders some standard characterization methods, such as Hall measurement, useless.

Hongxing Jiang, Jingyu Lin and colleagues have developed a deep-UV laser spectroscopy system that allows them to make picosecond time-resolved photoluminescence measurements of AlN and, thus, to characterize and thereby improve the material's quality. The system uses a 3-mW, 196-nm frequency-quadrupled Ti:sapphire laser that excites the sample. A streak camera captures the resulting exciton photoluminescence. Because only high-quality semiconductor materials emit exciton photoluminescence, Jiang explained, the spectra reveal the optical quality of the sample.

The group measured the photoluminescence spectra from 2.2 to 6.2 eV in AlN at 10 K. The ratio of the intensity of signals caused by impurities to the signal from the band-edge emission at 6.033 eV reveals the optical quality of the compound. If the emission from the defects and impurities in the compound does not overpower that of the compound itself, the optical quality is sufficient. The researchers found that this ratio is directly related to the growth conditions.

Determining the quality of the material is key to improving the manufacturing process. Because AlN and many of its III-nitride cousins are grown on foreign substrates, defects, dislocations and impurities are a significant problem. The new work indicates that it is possible to make AlN that displays less thermal quenching of the emission intensity and fewer problems resulting from impurities, dislocations and nonradiative recombination channels than GaN.

Much work remains to be done before manufacturers can incorporate AlN-based UV lasers in their products. Nevertheless, the group is taking the initial steps to developing N- and P-type AlN. Other studies already have suggested that N-type AlN can be created with dopants such as silicon.